arm: fpu: Add '.arch_extension fp' to fpu macros

Since the tests are built with '-mgeneral-regs-only', clang-18 tends to
push 'q' registers out of the scope and hence, the following error is
observed:

arm/fpu.c:281:3: error: instruction requires: fp-armv8
  281 |                 fpu_reg_write(*indata);
      |                 ^
arm/fpu.c:74:15: note: expanded from macro 'fpu_reg_write'
   74 |         asm volatile("ldp q0, q1, [%0], #32\n\t"        \
      |                      ^
<inline asm>:1:2: note: instantiated into assembly here
    1 |         ldp q0, q1, [x8], #32
      |         ^

Hence, explicitly add fp support where these registers are used.

Reported-by: Andrew Jones <andrew.jones@linux.dev>
Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
Signed-off-by: Andrew Jones <andrew.jones@linux.dev>