Add ARM Neon vmvn_*/vmvnq_* bitwise not intrinsics

[jmaibaum]

Hi,

I wanted to contribute to #148 by adding a family of ARM Neon intrinsics. I started with the vmvn_{s,u}* / vmvnq_{s,u}* family of intrinsics, which are all included in this PR, including tests.

I just found out that there are also the vmvn_p8 and vmvnq_p8 "poly" intrinsics which I did not yet implement. I would most likely want to add them as well before this PR should be merged. But, as this is my first contribution to the Rust standard libraries, I wanted to ask for comments/review first before continuing. So please let me know, if anything could/should be done different/better.

This has been tested locally on a Raspberry Pi 3 B+ for the armv7hf part. All new tests pass, except assert_vmvn_s8_vmvn. I might be suffering from some form of #565 , please see my comment over there.

[gnzlbg]

This LGTM. Please ping me once CI is green - the wasm32 failure is spurious.

[jmaibaum]

Added missing vmvn_p8/vmvnq_p8, which completes the vmvn* family of ARM Neon intrinsics.

Rebased on top of current master and thus force-pushed.

Local tests and codegen checks on RaspberryPi 3 B+ were all green now, crossing my fingers for CI.

[jmaibaum]

@gnzlbg CI passed now, except for the following 3 Jobs:

• mips-unknown-linux-gnu - build-only

• wasm32-unknown-unknown (did not look into these two)

• aarch64-android-linux - no assert_instr (Travis seems to have missed the output from the Android emulator here, errored after not receiving any output for 10mins)

[gnzlbg]

Could you please rebase this on top of master?

[jmaibaum]

@gnzlbg Sure.

[gnzlbg]

Thanks!