Explicitly specify -finput-charset=UTF-8 for GCC/Clang

...after 9a7d2d72 "tell msvc our source code is
written using utf-8" did the equivalent for MSVC.  That should cause the C++
string literal u8"ßa" (in vcl/qa/cppunit/mnemonic.cxx) introduced with that
commit to reliably be interpreted as intended by all our toolchains.

Change-Id: Ibbda8588a3ca66d1c2764f70aa999fad243f1bb1

solenv/gbuild/platform/com_GCC_defs.mk

@@ -55,6 +55,7 @@ gb_CFLAGS_COMMON := \
 	-Wstrict-prototypes \
 	-Wundef \
 	-Wunused-macros \
+	-finput-charset=UTF-8 \
 	-fmessage-length=0 \
 	-fno-common \
 	-pipe \
@@ -67,6 +68,7 @@ gb_CXXFLAGS_COMMON := \
 	-Wextra \
 	-Wundef \
 	-Wunused-macros \
+	-finput-charset=UTF-8 \
 	-fmessage-length=0 \
 	-fno-common \
 	-pipe \