MinGW: also copy libmingwex.a from C toolchain

[mati865]

Fixes recent CI error:

note: C:\Program Files (x86)\Rust\lib\rustlib\i686-pc-windows-gnu\lib/libmsvcrt.a(dyims01169.o):(.text+0x0): multiple definition of `longjmp'
          C:\Program Files (x86)\Rust\lib\rustlib\i686-pc-windows-gnu\lib/libmingwex.a(lib32_libmingwex_a-mingw_getsp.o):mingw_getsp.S:(.text+0x5): first defined here

Successful build: https://ci.appveyor.com/project/mati865/libc/builds/26425673
It also should be added to Azure in #1376

[rust-highfive]

r? @gnzlbg

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[gnzlbg]

@bors: r+

[bors]

📌 Commit 63fd0f5 has been approved by gnzlbg

[bors]

⌛ Testing commit 63fd0f5 with merge 1372eb2...

[mati865]

All green.
I think @pietroalbini redeployed RCS and bors died.

[gnzlbg]

@bors: retry

[bors]

⌛ Testing commit 63fd0f5 with merge 04d7ccc...

[bors]

💥 Test timed out

[gnzlbg]

@bors: r+

[bors]

💡 This pull request was already approved, no need to approve it again.

• This pull request previously failed. You should add more commits to fix the bug, or use retry to trigger a build again.
• There's another pull request that is currently being tested, blocking this pull request: Add support for FreeBSD CURRENT (aka freebsd13) #1440

[bors]

📌 Commit 63fd0f5 has been approved by gnzlbg

[mati865]

If #1376 is close to being merged you can close this one.

[gnzlbg]

I'm closing this in favor of that one, but bors is timing out all over the place all the time.