Add initial x86_64 profile

profiles/arch/x86_64/make.defaults

-ARCH=x86_64
+# We definitely use 64-bit ABI.
+ABI_X86="64"
+IUSE_IMPLICIT="abi_x86_64"
+
+ARCH="x86_64"
+ACCEPT_KEYWORDS="${ARCH} amd64"  # HACK for using Gentoo ebuilds
+
+# -O2           -> Perform second-level optimisations.
+#                  Not -Os because most/all x86_64 aren't starved for space.
+# -ggdb         -> Generate GDB debugging information.
+#                  This is used with splitdebug to make -dbg split packages.
+# -march=nocona -> Require a "Nocona" Pentium 4 or newer.
+#                  At some point, pending user demand, this may change to core2.
+#                  !!!NOTE!!! If that happens, Nocona P4 will no longer work!
+# -mtune=core2  -> Tune for Core 2 processors.
+#                  Most tuning for Core 2 is applicable to * Bridge and *well,
+#                  and makes code more performant on the much larger base of
+#                  those machines.
+# -fno-omit-... -> Never omit the frame pointer.
+#                  This is crucial for proper debugging.
+# -mfpmath=sse  -> Use SSE for all floating point math.
+#                  Since SSE is always available, this knob ensures that GCC
+#                  will take advantage of it over preferring 387 math, which
+#                  can drag down performance on modern x86_64.
+CFLAGS="-O2 -ggdb -march=nocona -mtune=core2 -fno-omit-frame-pointer -mfpmath=sse"
+CXXFLAGS="${CFLAGS}"
+
+ELIBC="musl"
+
+USE="pam threads"