Add "ax" flags to .section directives in assembly

This commit fixes a linking error when using LLVM's lld.

llvm-as doesn't assume or assign section flags for input section if
not specified. This will make the section non-allocatable and put
into a none segment and trigger relocation linking errors against
symbols in non-allocatble sections.

For example, when building with LLVM/lld for AArch64, the following
error occurs:
(traps.S.obj:(function arm_vector_table: .vectors+0x0): has non-ABS
relocation R_AARCH64_JUMP26 against symbol 'invalid_vector_entry')
This does not happen with ld.bfd (GNU's linker) as it seems to allow
relocations against symbols in non-allocatable sections.

The GNU's assembler documentation states that:
"If no flags are specified, the default flags depend upon the
section name. If the section name is not recognized, the default
will be for the section to have none of the above flags: it will
not be allocated in memory, nor writable, nor executable.
The section will contain data. [1]"

This commit explicitly sets .section flags in assembly to avoid
this error and for better intentionality (and good practice)
without relying on toolchains handling flags and linkage differently.

[1] https://sourceware.org/binutils/docs/as/Section.html

Sponsored by: DARPA.

Signed-off-by: Hesham Almatary <hesham.almatary@cl.cam.ac.uk>

src/arch/arm/64/head.S

@@ -75,7 +75,7 @@
 #define SCTLR   sctlr_el1
 #endif
 
-.section .boot.text
+.section .boot.text, "ax"
 BEGIN_FUNC(_start)
     /* Save x4 and x5 so we don't clobber it */
     mov     x7, x4

src/arch/arm/64/traps.S

@@ -43,7 +43,7 @@
     b       \label
 .endm
 
-.section .vectors
+.section .vectors, "ax"
 
 BEGIN_FUNC(arm_vector_table)
     ventry  invalid_vector_entry           // Synchronous EL1t/EL2t
@@ -67,7 +67,7 @@ BEGIN_FUNC(arm_vector_table)
     ventry  invalid_vector_entry           // SError 32-bit EL0/EL1
 END_FUNC(arm_vector_table)
 
-.section .vectors.text
+.section .vectors.text, "ax"
 
 .macro kernel_enter
     /* Storing thread's stack frame */

src/arch/riscv/traps.S

@@ -14,7 +14,7 @@
 
 #define REGBYTES (CONFIG_WORD_SIZE / 8)
 
-.section .text
+.section .text, "ax"
 
 .global trap_entry
 .extern c_handle_syscall

src/arch/x86/32/head.S

@@ -14,7 +14,7 @@
 #include <config.h>
 #include <machine/assembler.h>
 
-.section .phys.text
+.section .phys.text, "ax"
 
 BEGIN_FUNC(enable_paging)
     # Set PSE (bit 4) to enable 4M pages

src/arch/x86/32/machine_asm.S

@@ -6,7 +6,7 @@
 
 #include <machine/assembler.h>
 
-.section .boot.text
+.section .boot.text, "ax"
 
 /* user-level selectors from </arch/mode/object/structures.h> */
 #define GDT_NULL    0

src/arch/x86/32/traps.S

@@ -79,7 +79,7 @@ int_##number:                                \
     SET_SELECTOR(fs, SEL_FS, tmp) \
     SET_SELECTOR(gs, SEL_GS, tmp)
 
-.section .text
+.section .text, "ax"
 
 INT_HANDLER_WITHOUT_ERR_CODE(00)
 INT_HANDLER_WITHOUT_ERR_CODE(01)

src/arch/x86/64/machine_asm.S

@@ -14,9 +14,9 @@
  * as integer type according to the ABI.
  */
 
-.section .boot.text
+.section .boot.text, "ax"
 
-.section .text
+.section .text, "ax"
 
 BEGIN_FUNC(out8)                        # port, value
     movq    %rdi, %rdx

src/arch/x86/64/traps.S

@@ -170,7 +170,7 @@ int_##number:                                       \
 #define INT_HANDLER_WITH_ERR_CODE(number,except) INT_HANDLER_COMMON(number,,except)
 #define INT_HANDLER_WITHOUT_ERR_CODE(number,except) INT_HANDLER_COMMON(number,pushq $0x0,except)
 
-.section .text
+.section .text, "ax"
 .code64
 
 INT_HANDLER_WITHOUT_ERR_CODE(00,1)