x86: Fix linking u-boot with ld.lld

x86: Fix linking u-boot with ld.lld

When linking the final u-boot binary with LLD, the following link errors
are seen:

ld.lld: error: can't create dynamic relocation R_386_32 against local
               symbol in readonly segment; recompile object files with
               -fPIC or pass '-Wl,-z,notext' to allow text relocations
               in the output
>>> defined in arch/x86/cpu/start.o
>>> referenced by arch/x86/cpu/start.o:(.text.start+0x32)
[...]
>>> defined in arch/x86/cpu/start16.o
>>> referenced by arch/x86/cpu/start16.o:(.start16+0x1C)

According to Nick Desaulniers:

"This is a known difference between GNU and LLVM linkers; the GNU
 linkers permit relocations in readonly segments (making them not read
 only), LLVM does not (by default)."

Since U-Boot apparently seems to use relocations in readonly segments,
change the global linker flags to permit them when linking with LLD by
specifying '-z notext'.

Signed-off-by: Alistair Delva <adelva@google.com>
Cc: Nick Desaulniers <ndesaulniers@google.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>