I tried to bootstrap from the source, nim-1.2.0.tar.xz, on a machine with gcc 4.8.2.
During linking I got undefined reference to __builtin_saddll_overflow errors so it seems that the C code for bootstrapping was not generated with -d:nimEmulateOverflowChecks (the workaround for generating code for old compilers, as specified in the 1.2.0 release notes).
It would be nice if the install page at least specified the actual minimum gcc requirement for compiling from nim-1.2.0.tar.xz. Alternatively a second source tarball could be provided that is generated with the above -d:nimEmulateOverflowChecks, which at least generates a working compiler (although slower) when using older gcc versions.
The frozen C sources from
git clone --depth 1 https://github.com/nim-lang/csources.git
should work to create a Nim 1.2 via -d:nimEmulateOverflowChecks. Not sure if we need to update our instructions given that GCC 4.8.2 is almost 7 years old now...