Revert "[Build] Build libc++ with gcc need libs atomic"

This reverts commit 0a99a45.

Reason for revert: Quoted other patches

Original change's description:
> [Build] Build libc++ with gcc need libs atomic
>
>   In commit 34259b add atomic.cpp into
> buildtools/third_party/libc++/trunk/src, so if using gcc to build we
> need add lib "atomic"
>
> Bug: 1184590
> Change-Id: I999893aa0105fd2ae172019881039b20e2540294
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2732971
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Reviewed-by: Nico Weber <thakis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#861041}

Bug: 1184590
Change-Id: Ifa0188d382b8f141417d56ae6808024ba3aadb88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2751228
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#862068}

build/build_config.h

@@ -22,14 +22,13 @@
 //  Processor:
 //    ARCH_CPU_ARM64 / ARCH_CPU_ARMEL / ARCH_CPU_MIPS / ARCH_CPU_MIPS64 /
 //    ARCH_CPU_MIPS64EL / ARCH_CPU_MIPSEL / ARCH_CPU_PPC64 / ARCH_CPU_S390 /
-//    ARCH_CPU_S390X / ARCH_CPU_X86 / ARCH_CPU_X86_64 / ARCH_CPU_RISCV64
+//    ARCH_CPU_S390X / ARCH_CPU_X86 / ARCH_CPU_X86_64
 //  Processor family:
 //    ARCH_CPU_ARM_FAMILY: ARMEL or ARM64
 //    ARCH_CPU_MIPS_FAMILY: MIPS64EL or MIPSEL or MIPS64 or MIPS
 //    ARCH_CPU_PPC64_FAMILY: PPC64
 //    ARCH_CPU_S390_FAMILY: S390 or S390X
 //    ARCH_CPU_X86_FAMILY: X86 or X86_64
-//    ARCH_CPU_X86_FAMILY: Riscv64
 //  Processor features:
 //    ARCH_CPU_31_BITS / ARCH_CPU_32_BITS / ARCH_CPU_64_BITS
 //    ARCH_CPU_BIG_ENDIAN / ARCH_CPU_LITTLE_ENDIAN
@@ -198,11 +197,6 @@
 #define ARCH_CPU_32_BITS 1
 #define ARCH_CPU_BIG_ENDIAN 1
 #endif
-#elif defined(__riscv) && (__riscv_xlen == 64)
-#define ARCH_CPU_RISCV_FAMILY 1
-#define ARCH_CPU_RISCV64 1
-#define ARCH_CPU_64_BITS 1
-#define ARCH_CPU_LITTLE_ENDIAN 1
 #else
 #error Please add support for your architecture in build/build_config.h
 #endif

buildtools/third_party/libc++/BUILD.gn

@@ -51,11 +51,6 @@ target(_libcxx_target_type, "libc++") {
   if (libcxx_is_shared) {
     no_default_deps = true
   }
-
-  if (is_linux && !is_clang) {
-    libs = [ "atomic" ]
-  }
-
   sources = [
     "trunk/src/algorithm.cpp",
     "trunk/src/any.cpp",