[Driver][Fuchsia] Make -mfix-cortex-a53-835769 default when targeting Fuchsia

[MaskRay]

In the future, such customizations can be done via configuration files (https://blogs.gentoo.org/mgorny/2022/10/07/clang-in-gentoo-now-sets-default-runtimes-via-config-file/).
I know driver logic is sometimes more convenient and you may want "first-class support" and I do not object.

[phosek]

LGTM

[phosek]

Shall we only apply this when CPU targeted is either generic or cortex-a53 akin to https://github.com/llvm/llvm-project/blob/b5d5813762b57a8b086ad485ec6e7e06f137fdcf/clang/lib/Driver/ToolChains/Fuchsia.cpp#L93?

[abrachet]

In D136703#3883403, @phosek wrote:

Shall we only apply this when CPU targeted is either generic or cortex-a53 akin to https://github.com/llvm/llvm-project/blob/b5d5813762b57a8b086ad485ec6e7e06f137fdcf/clang/lib/Driver/ToolChains/Fuchsia.cpp#L93?

Sure

clang/lib/Driver/ToolChains/Arch/AArch64.cpp

if (Args.hasArg(options::OPT_mno_neg_immediates))
     Features.push_back("+no-neg-immediates");
 
   if (Arg *A = Args.getLastArg(options::OPT_mfix_cortex_a53_835769,
                                options::OPT_mno_fix_cortex_a53_835769)) {
     if (A->getOption().matches(options::OPT_mfix_cortex_a53_835769))
       Features.push_back("+fix-cortex-a53-835769");
     else
       Features.push_back("-fix-cortex-a53-835769");
-  } else if (Triple.isAndroid()) {
-    // Enabled A53 errata (835769) workaround by default on android
+  } else if (Triple.isAndroid() || Triple.isOSFuchsia()) {
+    // Enabled A53 errata (835769) workaround by default on Android and Fuchsia
     Features.push_back("+fix-cortex-a53-835769");
   }
 
   if (Args.getLastArg(options::OPT_mno_bti_at_return_twice))
     Features.push_back("+no-bti-at-return-twice");
 }

clang/test/Driver/aarch64-fix-cortex-a53-835769.c

 // RUN: %clang -target aarch64-linux-eabi %s -### 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-DEF %s
 // RUN: %clang -target aarch64-linux-eabi -mfix-cortex-a53-835769 %s -### 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-YES %s
 // RUN: %clang -target aarch64-linux-eabi -mno-fix-cortex-a53-835769 %s -### 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-NO %s
 
 // RUN: %clang -target aarch64-android-eabi %s -### 2>&1 \
 // RUN:   | FileCheck --check-prefix=CHECK-YES %s
 
+// RUN: %clang --target=aarch64-fuchsia %s -### 2>&1 \
+// RUN:   | FileCheck --check-prefix=CHECK-YES %s
+
 // CHECK-DEF-NOT: "{[+-]}fix-cortex-a53-835769"
 // CHECK-YES: "+fix-cortex-a53-835769"
 // CHECK-NO: "-fix-cortex-a53-835769"