Align ClearShadowForThreadStackAndTLS for NetBSD/i386

Summary:
The static TLS vector for the main thread on NetBSD/i386 can be
unaligned in terms of the shadow granularity. Align the start of it with
Round Down and end of it with Round Up operations for the shadow
granularity shift.

Example static TLS vector ranges on NetBSD/i386:
tls_begin_=0xfbee7244 tls_end_=0xfbee726c.

ClearShadowForThreadStackAndTLS() is called from the Main Thread
bootstrap functions.

This change restores the NetBSD x86 32-bit (i386) support.

Sponsored by <The NetBSD Foundation>

Reviewers: vitalybuka, joerg

Reviewed By: vitalybuka

Subscribers: kubamracek, llvm-commits, #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D46585

llvm-svn: 332792