grub: add -Wno-error=trampolines to native CFLAGS

grub: add -Wno-error=trampolines to native CFLAGS

(Resend as mailer munged previous submission. Also updated comments
to indicate problem is not limited to gcc 5.3.0.)

Adds -Wno-error=trampolines to native CFLAGS prevent multiple compile
errors when using gentoo builf of gcc.

[YOCTO #9201]

(From OE-Core rev: c2450e4fd01487aaf5bb37a4c70a5397650ba5f8)

Signed-off-by: Bill Randle <william.c.randle@intel.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>