LLVMReactor: Remove "stack-probe-size" fudge, bind _chkstk extern
LLVMReactor: Remove "stack-probe-size" fudge, bind _chkstk extern

Fixes a number of dEQP tests that use over 4kB's of allocas on Win32.

Bug: b/135866359
Change-Id: I0e50eea6a70d284c8d653fb070f4ae982c02914c
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/33213
Tested-by: Ben Clayton <1f693eb7158dc33015048c09d6712f74dabddf79@google.com>
Kokoro-Presubmit: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>

google/swiftshader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader