Documentation: raise minimum supported version of GCC to 5.1

commit fad7cd33 ("nbd: add the check to prevent overflow in
__nbd_ioctl()") raised an issue from the fallback helpers added in
commit f0907827 ("compiler.h: enable builtin overflow checkers and
add fallback code")

Specifically, the helpers for checking whether the results of a
multiplication overflowed (__unsigned_mul_overflow,
__signed_add_overflow) use the division operator when
!COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW.  This is problematic for 64b
operands on 32b hosts.

Also, because the macro is type agnostic, it is very difficult to write
a similarly type generic macro that dispatches to one of:
 * div64_s64
 * div64_u64
 * div_s64
 * div_u64

Raising the minimum supported versions allows us to remove all of the
fallback helpers for !COMPILER_HAS_GENERIC_BUILTIN_OVERFLOW, instead
dispatching the compiler builtins.

arm64 has already raised the minimum supported GCC version to 5.1, do
this for all targets now.  See the link below for...