fix(build): allow warnings when using lld

After https://reviews.llvm.org/D118840, ld.lld reports
below section type mismatch warnings:

  LD      trusted-firmware-a/build/qemu/release/bl1/bl1.elf
ld.lld: error: section type mismatch for base_xlat_table
>>> trusted-firmware-a/build/qemu/release/bl1/xlat_tables_context.o:
    (base_xlat_table): SHT_PROGBITS
>>> output section .bss: SHT_NOBITS

ld.lld: error: section type mismatch for xlat_table
>>> trusted-firmware-a/build/qemu/release/bl1/xlat_tables_context.o:
    (xlat_table): SHT_PROGBITS
>>> output section xlat_table: SHT_NOBITS

So allow warnings to make build pass.

Change-Id: I70503ad03b3be7a18ffe29ef6f2127c3f4d540f1
Signed-off-by: Yabin Cui <yabinc@google.com>
diff --git a/Makefile b/Makefile
index f4d623e..8f859fc 100644
--- a/Makefile
+++ b/Makefile
@@ -468,12 +468,14 @@
 
 # LD = gcc-ld (ld) or llvm-ld (ld.lld) or other
 else
-TF_LDFLAGS		+=	--fatal-warnings -O1
+TF_LDFLAGS		+=	-O1
 TF_LDFLAGS		+=	--gc-sections
 # ld.lld doesn't recognize the errata flags,
-# therefore don't add those in that case
+# therefore don't add those in that case.
+# ld.lld reports section type mismatch warnings,
+# therefore don't add --fatal-warnings to it.
 ifeq ($(findstring ld.lld,$(notdir $(LD))),)
-TF_LDFLAGS		+=	$(TF_LDFLAGS_$(ARCH))
+TF_LDFLAGS		+=	$(TF_LDFLAGS_$(ARCH)) --fatal-warnings
 endif
 endif