boot: Ensure ARM UEFI binary does not contain FP/SIMD instructions

ARM toolchains will sometimes optimize otherwise floating-point-free
code with floating point and SIMD instructions.  This was happening with
systemd-bootarm.efi and it was causing U-Boot to crash and reset the
CPU.  U-Boot does not support the ARM VFP floating point coprocessor,
which is an optional piece of hardware anyway [1].

Ensure the compiler does not generate FP/SIMD instructions by supplying
the `-mgeneral-regs-only` option when building for ARM [2].

The other option you often see to solve these problems is
`-msoft-float`, but that changes the ABI and prevents linking with
libgnuefi.

[1] https://lists.denx.de/pipermail/u-boot/2011-February/087736.html
[2] https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html#index-mgeneral-regs-only-1

(cherry picked from commit 1ad60562)
(cherry picked from commit 73678d23)
(cherry picked from commit 869614a5)