[release/3.1] Stop leaking atoll in pal and fix unicode header lookup

[hoyosjs]

This fixes the build in newer OSX SDKs. @vseanreesermsft reported the following issue under SDK 11.3:

2021-11-17T17:14:20.3631480Z Executing make install -j 4
2021-11-17T17:14:20.4256230Z [  0%] Generating dummy/eventprovdotnetruntime.cpp, dummy/eventprovdotnetruntimerundown.cpp, dummy/eventprovdotnetruntimestress.cpp, dummy/eventprovdotnetruntimeprivate.cpp
2021-11-17T17:14:20.4529460Z [  0%] Building C object src/corefx/System.Globalization.Native/CMakeFiles/System.Globalization.Native.dir/pal_calendarData.c.o
2021-11-17T17:14:20.4707690Z [  0%] Building C object src/corefx/System.Globalization.Native/CMakeFiles/System.Globalization.Native_Static.dir/pal_calendarData.c.o
2021-11-17T17:14:20.4710540Z Scanning dependencies of target coreclrpal
2021-11-17T17:14:20.5081520Z [  0%] Building CXX object src/pal/src/CMakeFiles/coreclrpal.dir/cruntime/file.cpp.o
2021-11-17T17:14:20.7220520Z In file included from /Users/runner/work/1/s/src/pal/src/cruntime/file.cpp:22:
2021-11-17T17:14:20.7322630Z In file included from /Users/runner/work/1/s/src/pal/src/include/pal/palinternal.h:620:
2021-11-17T17:14:20.7422970Z In file included from /Applications/Xcode_13.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.3.sdk/usr/include/c++/v1/stdlib.h:93:
2021-11-17T17:14:20.7526650Z /Applications/Xcode_13.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX11.3.sdk/usr/include/stdlib.h:139:3: error: 'atoll' is missing exception specification 'throw()'
2021-11-17T17:14:20.7625410Z          atoll(const char *);
2021-11-17T17:14:20.7727310Z          ^
2021-11-17T17:14:20.7829290Z /Users/runner/work/1/s/src/pal/inc/pal.h:4224:33: note: previous declaration is here
2021-11-17T17:14:20.7930400Z PALIMPORT long long int __cdecl atoll(const char *) THROW_DECL;

[hoyosjs]

This is a build of that leg https://dev.azure.com/dnceng/internal/_build/results?buildId=1475498&view=logs&j=66a6fde4-8762-5649-69e2-c0253dc61126&t=7c509286-d793-5ea5-6757-a418e93ea8b7

[jeffschwMSFT]

@safern is it safe to merge these now?

[jeffschwMSFT]

Approved. This is an infra change and we should treat as tell mode.

[safern]

Branch is closed till Demeber 17th after the branding changes are made. I will mark as no-merge and will merge whenever the branch is open. Thanks for asking.

[safern]

I didn't realize this was blocking the official builds for 3.1. I will go ahead and merge it.