Rollup merge of #48085 - alexcrichton:update-dlmalloc, r=Mark-Simulacrum
Rollup merge of #48085 - alexcrichton:update-dlmalloc, r=Mark-Simulacrum

Update the dlmalloc submodule

A bug was recently fixed in dlmalloc which meant that released memory to the
system accidentally wasn't getting reused, causing programs to be far slower
than they should be!

aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,GBGamer/rust