runtime: do not create new threads during malloc. 	the signal handling stack is a different size than 	the normal stack, so it cannot be allocated using 	the backup stack allocator.
runtime: do not create new threads during malloc.
	the signal handling stack is a different size than
	the normal stack, so it cannot be allocated using
	the backup stack allocator.

Fixes issue 250.

R=agl1
CC=golang-dev
http://codereview.appspot.com/157044

apparentlymart/golang,h8liu/golang,tv42/old-go,pombredanne/go-deleteme,ssrl/go,tianon/debian-golang,dsriram/golang,dsriram/golang,bdwalton/debian-golang,dsriram/golang,nightlyone/golang,bdwalton/debian-golang,bdwalton/debian-golang,bdwalton/debian-golang,tianon/debian-golang,dsriram/golang,gnanderson/go,gnanderson/go,ssrl/go,tv42/old-go,apparentlymart/golang,gnanderson/go,tv42/old-go,bdwalton/debian-golang,apparentlymart/golang,pombredanne/go-deleteme,nightlyone/golang,nightlyone/golang,nightlyone/golang,h8liu/golang,apparentlymart/golang,gnanderson/go,bdwalton/debian-golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,ssrl/go,h8liu/golang,h8liu/golang,tianon/debian-golang,apparentlymart/golang,tianon/debian-golang,dsriram/golang,tianon/debian-golang,h8liu/golang,gnanderson/go,ssrl/go,tv42/old-go,dsriram/golang,tianon/debian-golang,h8liu/golang,pombredanne/go-deleteme,tianon/debian-golang,nightlyone/golang,h8liu/golang,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,nightlyone/golang,gnanderson/go,tianon/debian-golang,gnanderson/go,ssrl/go,apparentlymart/golang,nightlyone/golang,ssrl/go,pombredanne/go-deleteme,bdwalton/debian-golang,bdwalton/debian-golang,ssrl/go,apparentlymart/golang