cortex-m/irq_handler: Replace inline asm with C

When building with clang and LTO is enabled, clang optimizes out
"routine" (the interrupt handler) and fails to link. gcc does not
optimize out the routine.

Rewriting IRQ_HANDLER in C makes it compile with both compilers, though
it uses two more instructions.

Before this change the generated assembly for an interrupt handler on
dartmonkey:

arm-none-eabi-objdump -d ./build/dartmonkey/RW/ec.RW.elf

081027a8 <irq_37_handler>:
 81027a8:       4670            mov     r0, lr
 81027aa:       b501            push    {r0, lr}
 81027ac:       f7ff ffea       bl      8102784 <uart_interrupt>
 81027b0:       e8bd 4001       ldmia.w sp!, {r0, lr}
 81027b4:       f009 bbc4       b.w     810bf40 <task_resched_if_needed>

After this change the generated assembly for an interrupt handler on
dartmonkey:

arm-none-eabi-objdump -d ./build/dartmonkey/RW/ec.RW.elf

081027e0 <irq_37_handler>:
 81027e0:       b507            push    {r0, r1, r2, lr}
 81027e2:       f8cd e004       str.w   lr, [sp, #4]
 81027e6:       f7ff ffe9       bl      81027bc <uart_interrupt>
 81027ea:       9801            ldr     r0, [sp, #4]
 81027ec:       b003            add     sp, #12
 81027ee:       f85d eb04       ldr.w   lr, [sp], #4
 81027f2:       f009 bbc5       b.w     810bf80 <task_resched_if_needed>

BRANCH=none
BUG=b:172020503
TEST=make buildall
TEST=./test/run_device_tests.py --board dartmonkey
     => PASS

Signed-off-by: Tom Hughes <tomhughes@chromium.org>
Change-Id: I7029556afc4a760e81885d8f84b7d787f48c4036
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3566661
Reviewed-by: Paul Fagerburg <pfagerburg@chromium.org>

0 files changed, 0 insertions, 0 deletions